1. Field of the Invention
The present invention relates to a hydraulic shock absorber of a damping power adjustable type to be mounted on a suspension system of a vehicle such as an automobile and the like.
2. Related Background Art
Among hydraulic shock absorbers to be mounted on a suspension system of a vehicle such as an automobile and the like, there are hydraulic shock absorbers of damping force adjustable type in which a damping force can be properly adjusted to improve comfort and/or steering stability in accordance with road conditions, running conditions and the like.
In general, the hydraulic shock absorber of this kind is designed so that a piston to which a piston rod is connected is slidably mounted within a cylinder filled with oil, two chambers defined by the piston in the cylinder are communicated with a main oil passage and a bypass passage, respectively, a damping force generating mechanism (orifice, disk valve and the like) for generating a great damping force is provided in the main oil passage, and a damping force generating mechanism for generating a small damping force and a damping force adjusting valve for opening and closing the bypass passage are provided in the bypass passage.
With this arrangement, when the damping force adjusting valve is opened, the sliding movement of the piston due to extension and contraction of the piston rod causes the oil in the cylinder to mainly flow into the bypass passage, thereby generating small damping forces both at the extension side and at the contraction side. In this case, the damping force has a soft property. On the other hand, when the damping force adjusting valve is closed, the sliding movement of the piston due to extension and contraction of the piston rod causes the oil in the cylinder to flow into the main oil passage alone, thereby generating large damping forces both at the extension side and at the contraction side. In this case, the damping force has a hard property. In this way, the damping force property can be changed by opening and closing the damping force adjusting valve.
When a vehicle is running under normal condition, by selecting the soft damping force property, the vibration of the vehicle caused due to the unevenness of the road surface can be absorbed to improve the comfort. On the other hand, when the vehicle is cornering, being accelerated or braked, or is running at a high speed, by selecting the hard damping force property, the change in posture of the vehicle can be suppressed to improve the steering stability. Further, there has been proposed a suspension control system wherein the comfort and steering stability can be improved by combining a control device and an actuator with the hydraulic shock absorber of damping force adjustable type to automatically change the damping force at real time in accordance with the road conditions and running conditions.
In such a suspension control system, when any combination of different damping force properties of the hydraulic shock absorber between the extension side and the contraction side (for example, combination of the hard property at the extension side and the soft property at the contraction side, or combination of the soft property at the extension side and the hard property at the contraction side) can be selected, the optimum damping force can be obtained quickly in accordance with the change in the road condition and/or running condition, thereby improving the comfort and the steering stability effectively.
To this end, conventionally, there have been proposed various hydraulic shock absorbers of damping force adjustable type in which two different bypass passages are provided at the extension side and the contraction side, respectively so that different (great and small) damping force properties can be obtained by adjusting cross-sectional areas of the bypass passages.
However, in such conventional hydraulic shock absorbers of the damping force adjustable type, since time cross-sectional area of the bypass passage set by the damping force adjusting valve is fixed, the resulting damping force is varied with the magnitude of time piston speed. Accordingly, the conventional damping force adjustment is accomplished by adjusting the damping coefficient on the basis of the piston speed. Thus, in order to directly control the damping force by the damping force control by using the above-mentioned suspension control system, the piston speed of the hydraulic shock absorber must be detected first, then, the damping coefficient for achieving the desired damping force associated with the piston speed must be calculated, and then, the actuator must be driven to open the damping force adjusting valve to an extent that the opening value of the valve corresponds to the calculated damping coefficient. Therefore, in the conventional techniques, there arose the problems that the control circuit is subjected to great load and that it is difficult to quickly change the damping force in accordance with the change in the road condition and/or running condition.